In a front projection system, a projector and a viewer are on the same side of a display surface, with an image from the projector reflecting from the display surface to the viewer. Front projection systems are typically on-axis projection systems in which the image is created and projected along a straight axis that is perpendicular to the display surface.
In a rear-projection system, a projector and a viewer are on the opposite sides of a screen. Rear projection television (RPTV) displays have found great consumer interest due to a large image size. A primary disadvantage of an RPTV system is a set volume and depth. In RPTV systems, minimizing the straight line projection path length by folding the optical path enables a cabinet housing a display surface and a projection light engine to be more compact. Thus, the optical path along which light propagates through these projection systems is typically folded with a large front surface mirror (FSM), known in the art as a fold mirror.
FIG. 1 provides an example of a prior art rear-projection system 1 comprising a light engine 3, a projection lens 5, a fold mirror 7, and a screen 9. The light engine 3 introduces light into the projection lens 5, which, in turn, projects the light onto the fold mirror 7 which reflects the light onto the screen 9.
In RPTV systems, images are composed by modulating the intensity of red, green, and blue components of light so as to form constantly changing images. As used herein, the term “component”, or a light beam, refers to a portion of a light transmission. For example, where a light transmission contains light of various wavelengths in the visible spectrum (e.g., blue, red, and green), the light transmission may be separated into a plurality of components, each corresponding to a range of wavelengths (i.e., color bands), such as blue, red, or green, in the visible spectrum. Further, a light transmission may comprise polarized light oriented in one or more planes.
For example, a polarization-based RPTV system is disclosed in U.S. Pat. No. 6,634,756 issued to Shimizu on Oct. 21, 2003, incorporated herein by reference. A rear-projection display system comprises a light source, a polarizing beam-splitter (PBS) that receives light from the light source, a projection lens that receives an image from the beam-splitter, a rear-projection screen, and a fold mirror that receives the image from the projection lens and reflects it onto the screen. In this system the beam-splitter having an inclined reflecting element is placed between an LCoS imager and a projection lens that projects the light onto a fold mirror whence it is reflected onto a screen. The beam-splitter receives an image, and reflects the image off the reflecting element at an angle of incidence that is substantially unequal to 45 degrees.
A part of an RPTV system, namely a polarization-based color management system, is disclosed in U.S. Pat. No. 6,678,015 in the names of Yi et al issued on Jan. 13, 2004 incorporated herein by reference. The systems comprises: a dichroic filter for allowing a selected one of light components, included in an incident light emitted from a light source, to be transmitted therethrough along a travel path of the incident light while reflecting the remaining light components in a direction perpendicular to the travel path of the incident light; a first synthesizing unit for receiving the light components reflected by the dichroic filter and separating the received light components from each other, the first synthesizing unit also serving to form images respectively corresponding to the separated light components via a first liquid crystal display (LCD) and a second LCD, to synthesize the images, and to allow the synthesized image to be directed in the same direction as the incident light introduced into the dichroic filter; a second synthesizing unit for receiving the light component transmitted through the dichroic filter, forming an image corresponding to the received light component via a third LCD, and reflecting the image in a direction perpendicular to the travel path of the incident light introduced into the dichroic filter; and a third synthesizing unit for synthesizing light beams respectively containing the images formed in the first and second synthesizing units, and allowing the synthesized light to be directed in the same direction as the incident light introduced into the dichroic filter. The first synthesizing unit comprises: a first color selecting retarder; a polarized beam splitter; the first LCD; the second LCD; and a second color selecting retarder. The second synthesizing unit comprises: a third color selecting retarder; a second polarized beam splitter; and a third LCD. The third synthesizing unit may comprises a third polarized beam splitter.
In optical systems using one or more mirrors that fold the beam path to the output screen, it is desired for the white light incident on the screen to be of uniform intensity and white uniformity across the entire viewing screen although, most often, it is not achieved. Quality of the image on the screen depends, in part, on reflectance of the fold mirror defined in the art as a ratio of energy of reflected to incident light. Besides properties of the mirror, a value of reflectance depends on properties of the incident light, in particular on an angle of incidence (AOI) of the light. In RPTV systems, different portions of a beam of light impinge on a fold mirror with largely varying angles of incidence causing variations in reflectance across the mirror surface. Furthermore, since a value of reflectance depends on polarization of incident light, reflectance for differently colored sub-beams, or components, varies differently contributing to image distortion. Thus special efforts need to be taken to provide uniform intensity and white color uniformity across the viewing screen.
There are known in the art methods of modifying a fold mirror in order to reduce image distortion:
U.S. Pat. No. 6,648,482 in the names of Hatakeyama et al issued on Nov. 18, 2003, discloses a polygon mirror comprising a reflecting surface and a single-layer film of uneven thickness, designed to ensure that variation in reflectance is small over a wide range of angles of incidence. The method of manufacturing the prism shaped polygon mirror having a film on the reflecting surface comprises a step of forming a coated film of a solution on the reflecting surface by rotating the polygon mirror, and the solution is applied only in the area of which the distance from the axis of rotation of the polygon mirror is at least Ri in the application step, where Ri is the minimum distance between the reflecting surface and the axis of rotation.
US Pat. Appl. No. 20040141157 by Ramachandran et al discloses an image projection system comprising a asymmetrically curved mirror for distortion compensation of an optical image.
US Pat. Appl. No. 20040057116 by Ogawa, discloses an optical filter comprising a substrate and an optical conversion film inclined with respect to the substrate by being continuously thinner from one end to another end, the optical conversion film comprising two types of thin films and having different refractive indices and being alternately stacked.
The aforementioned references are all incorporated herein by reference.
Although the aforementioned methods of altering the fold mirror by using a curved reflective surface and/or uneven covering layers appear to perform their intended function, they provide elaborate and expensive solutions.
It is therefore an object of this invention to provide a simple and cost-effective method for image distortion reduction in a polarization-based rear-projection system.
It is another object of this invention to increase a white balance across the viewing screen by lessening the intensity difference between light of differing orthogonal polarizations reflected from the fold mirror in an optical system.
It is another object of this invention to increase the visible white balance across the viewing screen by lessening the variation in intensity of green light over a range of angles of incidence.
It is a further object of this invention to provide a mirror with a coating designed to lessen a perceived variation in white color balance of light reflected across the coating wherein the white light is comprised of sub-beams of orthogonal polarization states.